This application is related to Japanese Patent Applications No. 2000-257524 filed on Aug. 28, 2000, No. 2001-120159 filed on Apr. 18, 2001, and No. 2001-137825 filed on May 8, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a vapor-compression refrigerant cycle in which a refrigerant pressure discharged from a compressor becomes equal to or larger than critical pressure of refrigerant. More particularly, the present invention relates to a maximum flow-amount control of refrigerant discharged from the compressor, and is suitably used for a vehicle air conditioner.
2. Description of Related Art
A compressor of a vehicle air conditioner is generally operated by driving force of a vehicle engine. Therefore, it is difficult to control a rotation speed of a compressor (i.e., a flow amount of refrigerant discharged from the compressor).
For example, in JP-A-58-105821, a variable-displacement compressor, in which a theoretical flow amount of refrigerant discharged from the compressor is variable, is used, and the theoretical flow amount discharged from the compressor is controlled in accordance with an engine rotation speed or a rotation speed of the compressor.
However, in a vapor-compression refrigerant cycle where a high-pressure side refrigerant pressure becomes equal to or higher than the critical pressure of refrigerant, the high-pressure side refrigerant pressure is high. For example, when carbon dioxide is used as refrigerant, the high-pressure side refrigerant pressure is 15 MPa. Accordingly, when the engine rotation speed increases, a consumed power of the compressor becomes larger, as compared with a vapor-compression refrigerant cycle using flon as refrigerant. Therefore, an engine load becomes larger, and travelling performance (e.g., acceleration performance) of the vehicle is deteriorated. In addition, when the engine rotation speed increases, the temperature of refrigerant discharged from the compressor becomes higher, and a thermal damage of the compressor or a rubber member may be caused.
In view of the foregoing problems, it is an object of the present invention to provide a refrigerant cycle for a vehicle air conditioner, which prevents a consumed power of a compressor from being excessively increased.
According to the present invention, in a refrigerant cycle for a vehicle air conditioner, a compressor is operated by power from an engine, and the compressor is disposed to suck and compress refrigerant so that refrigerant discharged from the compressor has a pressure equal to or higher than critical pressure of refrigerant. In the refrigerant cycle, a control unit is disposed to prevent a flow amount of refrigerant discharged from the compressor from becoming larger than a predetermined value. Because the flow amount of refrigerant discharged from the compressor is made not larger than the predetermined value, it can prevent a high-pressure side refrigerant pressure and a high-pressure side refrigerant temperature from being excessively increased with an increase of a rotation speed of the engine. Accordingly, the refrigerant cycle operates while preventing a consumed power of the compressor from being excessively increased, and it can prevent the compressor from being damaged.
Preferably, the compressor is a variable-displacement compressor in which a theoretical flow amount of refrigerant discharged from the compressor is variable, and the control unit controls the theoretical flow amount of refrigerant discharged from the compressor to prevent the flow amount of refrigerant discharged from the compressor from being larger than the predetermined value. Further, a fixed throttle is provided at a refrigerant discharge side of the compressor, and the theoretical flow amount is controlled based on a pressure difference between upstream and downstream sides of the fixed throttle in a refrigerant flow direction. Because the theoretical flow amount is controlled based on the pressure difference between upstream and downstream sides of the fixed throttle, the flow amount of refrigerant discharged from the compressor is not reduced even when an outside heat exchanger is frosted. Accordingly, it can prevent heat-absorbing capacity (heating capacity) of the outside heat exchanger from being reduced in a heating operation, when the outside heat exchanger is frosted. More preferably, the control unit is set in such a manner that the flow amount of refrigerant discharged from the compressor has an upper limit. Therefore, by suitably setting the upper limit, the consumed power of the compressor can be reduced, while the refrigerant cycle is normally operated.